From a technical standpoint it would be a case of internal Duplicate Content if the same content can be accessed through both a HTML-file as well as a PDF-document on your website. It would be external Duplicate Content if, for example, you offered a downloadable PDF version of the user-manual for every product in your online shop, while the same information is also available on the product manufacturer’s website.
Google says that, in the case of internal Duplicate Content, they usually prefer and rank the HTML version. If this scenario does not happend too often on your website, you usually do not have to worry about it.
You generally do not need to worry about duplicate content in a situation like this, even if you decide to mirror the content of your PDFs on HTML pages. If we recognize the URLs as containing duplicate content, we’ll just show one of them to users when they search; your site generally wouldn’t have any disadvantage by doing this.– John Mueller, Webmaster Trends Analyst, Google Switzerland
If Google were to show a duplicate content warning in the Google Search Console (GSC) under the “HTML-improvements” menu, for example, you could block the PDF document through the robots.txt file for your website and thereby keep Google-Bot from crawling the PDF. Alternatively, you can exclude the PDF file from being indext by using the x-robots-tag in the HTTP header. For more information, please see:
- Please keep in mind: If you block a URL in the robots.txt, it may still appear in the search results.
In the case of the external Duplicate Content in the example above, it is advisable to use a rel=”canonical” in the HTTP header of the PDF file with the original content as the source. Additional information can be found at: http://googlewebmastercentral.blogspot.de/2011/06/supporting-relcanonical-http-headers.html
Should PDF files really be crawled and indexed?
If you are using PDF files on your website, you should always ask yourself whether you want to primarily rank with them. If not, you should exclude these files from being indexed by Google-Bot in regard to the crawling- & index-budget of your website.